The ING Meteorological Archive stores the recordings of meteorological parameters like the temperature, the wind speed or the humidity from 1999. You can also find the yearly and the monthly variation of weather downtime (when the levels of cloudiness, wind speed, humidity, dust or dew excess the operational limits) in the ING Biennial Report's chapter on the use of observing time. As an average, the typical yearly weather downtime at the William Herschel Telescope is 25.3%.

Averaged monthly weather downtime 1989-2006 as measured at the William Herschel Telescope.

Information on the seeing characteristics of the La Palma site can be found on the ING seeing web pages. The IAC's Sky Quality Group studies the different characteristics of the quality of the sky above the Roque de Los Muchachos Observatory.

The Mercator Telescope provides half-hourly measurements of site extinction at night. The Carlsberg Meridian Telescope records the extinction nightly.

Atmospheric extinction above the observatory on La Palma is typically 0.12 mag in V band, and varies with wavelength. During July, August, September, a pall of neutral-extinction ('grey') dust hangs over the islands, and the total extinction may be as high as 1 mag.

The median moonless night sky brightness above La Palma at high elevation, high galactic latitude and high ecliptic latitude, at sunspot minimum, is B = 22.7, V = 21.9, R = 21.0, rms 0.1 mag/arcsec². The sky brightness in U and I is less well-determined, U approx 22.0 mag/arcsec² (few data), I approx 20.0 mag/arcsec² (variable). As at other dark sites, the main contributions to sky brightness are airglow and zodiacal light, in the ratio 2.5:1 at high ecliptic latitude.

• The sky is brighter at low ecliptic latitude (by 0.4 mag); at solar maximum (by 0.4 mag); and at high airmass (0.25 mag brighter at airmass 1.5).
• The mean brightness of the sky varies by < 0.1 mag with time after astronomical twilight.
• Light pollution is visible to the naked eye at certain azimuths on the horizon, but its contribution to the continuum brightness at the zenith is < 0.03 mag in all bands, meeting one of the IAU's two recommendations for a dark site. The NaD emission brightens the sky in both V and R broad bands by about 0.07 mag. Total contamination (line + continuum) at the zenith is < 0.03 mag in U, approx 0.02 mag in B, approx 0.10 mag in V, and approx 0.10 mag in R. A night-sky spectrum is available.
• The brightness of the sky shows no dependence on atmospheric extinction A_V, for A_V < 0.25 mag (as is the case on 80% of nights). The sample includes two nights with A_V > 0.25 mag. With A_V = 0.4 mag, the sky was brighter by approx 0.3 mag, and with A_V = 0.8 mag by approx 1.4 mag, probably because of enhanced back-scattering of streetlighting by the dust layer.

More information can be found in the following references:

• C. R. Benn, S. Ellison, 1998, "La Palma night-sky brightness", ING Technical Note, 115.
  
• M. Pedani, 2005, "An Updated View of the Light Pollution at the Roque de los Muchachos Observatory", ING Newsl., 9, 28.